1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium used as a magnetic disc.
2. Description of the Related Art
Recently, with progress of personal computers and work stations, the hard disc has been required to have a large capacity and small size, i.e., a high density. However, in order to realize a high recording density in the conventional longitudinal direction recording method, there are various problems. For example, if the recording bit is made smaller, there arises a problem of thermal fluctuation of recording magnetization and a problem of high coercive force which may exceed the recording capability of the recording head. To cope with this, a perpendicular magnetic recording method has been studied as means to significantly increase the recording density.
FIG. 156 is a cross sectional view of such a conventional magnetic recording medium. In this perpendicular magnetic recording medium 50, a perpendicular magnetization film 54 having a perpendicular magnetic anisotropy is formed on a substrate 56. For example, CoCr alloys are used for the perpendicular magnetization film (Journal of Magn. Soc. Japan, Vol. 8, No. 1, 1984, pp. 17–22).
However, in such a conventional perpendicular magnetic recording medium, there has been a problem that medium noise characteristic is very bad in a low recording density region. This is because the perpendicular magnetization film 54 is magnetized perpendicularly, and a demagnetizing field caused by the magnetic poles generated on the medium surface generates a reversed-magnetic domain. The lower is the recording density, the more the reversed-magnetic domains are generated. This has been the main cause to deteriorate the medium noise characteristic in the low recording density region. This medium noise increase in the low recording density region becomes a big trouble when forming a high-density information recording apparatus.
In order to reduce the effect of the demagnetizing field generated by the magnetic pole generated on the medium surface, there has been suggested to provide a soft magnetic layer under the perpendicular magnetization film so as to reduce the magnetic poles generated at the boundary between the perpendicular magnetization film and the soft magnetic layer (Japanese Patent Publication (examined) B58-91). This is generally known as a perpendicular two-layered magnetic recording medium.
However, in this two-layered perpendicular magnetic recording medium, if a perpendicular magnetization film is provided on a soft magnetic layer such as NiFe (Permalloy), there arises a problem that the soft magnetic layer generates a spike-shaped noise, disabling to obtain a preferable medium S/N ratio.
To cope with this, Japanese Patent Publication (unexamined) A59-127235, Japanese Patent Publication (unexamined) A59-191130, Japanese Patent Publication (unexamined) A60-239916, Japanese Patent Publication (unexamined) A61-8719, and Japanese Patent Publication (unexamined) A1-173312 suggest use of a perpendicular magnetization film on a backing layer made from Co or a Co alloy which is more advantageous than use of the permalloy soft magnetic layer.
However, the inventor of the present invention has found that when these soft magnetic films are used, these films easily absorb an external magnetic field generated by a magnetic disc rotation spindle motor. This results in concentration of the magnetic flux in a magnetic head and losing of recording signals. That is, the perpendicular magnetic recording medium of the two-layered film configuration can reduce the effect of the demagnetizing field caused by the magnetic poles generated on the medium surface, but this cannot be a solution for medium noise reduction.